Hexanolactam is an important starting material for manufacturing polyamide (also referred to as nylon). Hexanolactam is conventionally prepared by a method comprising reacting hydroxylamine with cyclohexanone to give cyclohexanone oxime, and then subjecting said cyclohexanone oxime to Beckman rearrangement to yield hexanolactam. Therefore, hydroxylamine is one of the important starting materials for hexanolactam production. Hydroxylamine is usually prepared by reducing nitric acid or its salt with hydrogen gas in presence of a mono metal catalyst such as palladium/carbon, or a double metal catalyst such as palladium-platinum carbon. However, pure hydroxylamine is unstable and decomposes automatically upon heating, which may result in explosion. So, hydroxylamine is usually prepared in a form its salt with an acid, such as hydroxylammonium sulfate, hydroxylammonium chloride, or hydroxylammonium phosphate, wherein hydroxylammonium phosphate is preferred.
Hydroxylamine is prepared in a gas/liquid/solid triple phase bubbling reaction system. As such a reaction system is a heterogeneous system, the reaction is affected not only by catalyst concentration, hydrogen pressure, hydrogen ion concentration, and the surface area of the catalyst, but also affected by mass transfer among gas, liquid and solid phases. The reaction scheme for preparation of hydroxylamine is as shown below:NH4NO3+2H3PO4+3H2NH3OHH2PO4+NH4H2PO4+2H2O
In the prior art, hydroxylamine was prepared, for example, in a reactor as shown in FIG. 1. Referring to FIG. 1, a reactor 10′ includes a reaction section 12′, a cooler 14′ disposed at the lower part of the reactor 10′, and a gas distributor 16′ disposed above the cooler 14′. An phosphate-buffered aqueous medium containing nitrate ions is fed to the reactor 10′ through a pipe 22′. Fresh hydrogen gas is delivered to a gas distributor 16′ of the reactor 10′ through a pipe 24′ and then introduced into the reaction section 12′. In the reaction section 12′, nitric acid or its salt contained in the phosphate-buffered aqueous medium is reduced by hydrogen gas to form hydroxylammonium phosphate. The aqueous medium containing the produced hydroxylammonium phosphate leaves the reactor 10′ through a pipe 30′. The unreacted hydrogen gas leaves the reactor 10′ through a pipe 32′ at the top of the reactor 10′, and is treated in a separator (not shown). The treated hydrogen gas after mixing with fresh hydrogen gas is circulated back to the gas distributor 16′ of the rector 10′, and reintroduced into the reaction section 12′.
In said method for preparation of hydroxylammonium phosphate, the hydrogen gas is fed into a reactor 10′ through a gas distributor 16′ at the middle part of the reactor, resulting in uneven distribution of hydrogen gas in the reactor 10′, and occurrence wall flow, channeling phenomenon etc., which adversely affects the efficiency of mass transfer among gas, liquid and solid phases during the reaction, and lowers the yield of hydroxylamine.
Therefore, it is desired to provide a method for preparing hydroxylamine in a gas/liquid/solid triple phase reaction system with high yield.